Liquid crystal displays (LCDs) are increasingly used in various display applications because they are thin, lightweight, and have low power consumption. Such LCDs have a structure in which a liquid crystal composition is confined between two substrates, at least one of which is transparent. The substrates are provided with an electrode layer on their internal faces (mutually facing surfaces) to impress an electric field on the liquid crystal composition and an alignment film in which an orientation treatment is performed that orients the liquid crystal. This orientation treatment serves to both orient and anchor the liquid crystal composition on the alignment film.
By applying an electric field exceeding the Freedericksz transition voltage between the electrodes of the two substrates, the orientation of the liquid crystal composition varies under the action of the electric field. Due to birefringence of the liquid crystal composition, these orientation variations change the optical properties of the display, and moreover by using polarizing plates it functions as a display device.
Such LCDs, called “classical” LCDs, have the following characteristics:
(1) When the external electric field is switched off after displaying information, the displayed information also disappears;
(2) the alignment film functions to align the liquid crystal molecules to be parallel with respect to the alignment film and has an extremely strong anchoring force with respect to the liquid crystal molecules. For that reason, even when an electric field is applied, the liquid crystal molecules in the vicinity of the alignment film basically maintain their parallel orientation with respect to the alignment film without reorienting to the direction of the electric field; and
(3) when the field is switched off, the state prior to the electric field application is reverted to.
A large amount of work has been done to improve the performances of the liquid crystal composition of classical LCDs by optimizing the physical properties including the temperature range, viscosity, elasticity, birefringence, dielectric anisotropy, Freedericksz transition voltage, etc. It is nearly impossible to optimize the above-mentioned characteristics with a single compound, and so mixtures combining a plurality of compounds are required (see Handbook of Liquid Crystals, Wiley-VCH Weinheim (1998)).
Furthermore, for these classical LCDs, the anchoring force with respect to the liquid crystal molecules of the alignment layer need not be strictly defined, with all that is required is that this anchoring be “strong”, in other words, greater than a given limiting value (LZ<15 nm, with LZ being defined below). Research has been conducted on this problem in obtaining strong anchoring in an alignment film, with several known alignment layer materials being used to provide strong anchoring (see Liquid Crystals—Applications and Uses, World Scientific Publishing Co. Pte. Ltd. Singapore (1990)).
In recent years research has been conducted on a new generation of nematic displays called “bistable” nematic displays. In these displays, the liquid crystal molecules have two stable textures without any applied voltage. The voltage is only applied for the time necessary to switch between these two orientation states. There is thus no need to keep applying a voltage in order to maintain the display. Due to its operating principle, this type of display consumes an amount of energy proportional to the number of image changes. Thus as the frequency of image changes drops, the power necessary for operation of the display tends towards zero. This type of display would therefore be extremely effective for mobile devices in which low power consumption is required.
Two kind of bistable displays have been proposed where the stable states are stabilized by the orienting film on the substrates. One kind uses orienting films which orient the molecules in two directions, i.e. bistable orienting films, the other kind uses more simple orienting films which orient in one direction only, i.e. monostable orienting films. The switching between the two stable states of these displays is obtained by breaking the anchoring of the molecules at least on one orienting film: an applied field put the molecules on the surface in a direction where the surface torque is zero and the energy maxima. After removing the field the molecules close to the film return in a stable orientation driving the molecules in the bulk to one or the other stable states.
The display device developed by the ZBD Displays Ltd. (G. P. Bryan-Brown et al., Nature, 399,338 (1999)) uses a bistable orienting film: close to the film, in one stable state, the molecules are oriented nearly parallel to the substrate; in the other stable state, the molecules are nearly perpendicular to the substrate. The Orsay Solid State Physic Laboratory proposed two bistable nematic displays using bistable orienting surfaces which orient the molecules in two tilted states: French Patent Application, Publication No. 2663770 where the commutation uses a flexoelectric effect and French Patent Application, Publication No. 2657699 which uses an electrochiral effect.
Two bistable nematic displays using monostable orienting films and commuting with anchoring breaking have been developed: the Bistable Nematic (BiNem®) display by Nemoptic Ltd. in France (French Patent Application, Publication Nos. 2740893 and 2740894, and U.S. Pat. No. 6,327,017) and the SBiND display developed by LICET Ltd. in Italy (European Patent Application, Publication No. 0 773 468, U.S. Pat. No. 5,995,173 and Japanese Unexamined Patent Application, Publication No. H09-274205).
The switching principle of the BiNem® bistable display by Nemoptic Ltd. is diagrammatically shown in FIG. 1. It uses two textures one texture is uniform or slightly twisted, texture T0, in which the liquid crystal molecules are approximately parallel to each other (±20°, and the other texture is T180 that differs from the first by a twist of 180°±20°. The nematic is chiralised with a spontaneous pitch p0, chosen to be close to four times the thickness d of the cell to equalize the energies of the two textures. In the absence of a voltage, these two states become minimum values in terms of energy. In the presence of a high voltage, anchoring of the molecules is broken on at least one of the substrates (specifically, on the alignment film on this substrate) and a nearly homeotropic orientation (H) of the liquid crystal molecules is obtained. This orientation state is a transition state (H), and can be switched to either of the two stable states (T0, T180). Slowly cutting the voltage can result in change to state T0 by elastic coupling between molecules close to the two surfaces, and quickly cutting the voltage will lead to state T180 by hydrodynamic coupling.
Bistable displays commuting by anchoring breaking require special properties of the liquid crystal mixture and the orienting film:                1) The anchoring on at least one alignment film has to be weak to allow the breaking by an applied field compatible with the driving electronics and the electrochemical properties of the different compounds of the nematic mixture.        2) The anchoring on the film can not be too weak because the liquid crystal textures in the stable states are stabilized by the anchoring. To maintain the textures, the anchoring torque need to be higher than the elastic torque applied on the surface by the bulk textures in the stable states.        3) The electrochemical stability of the different compounds of the mixture has to be higher to that for classical LCD displays. Indeed in classical LCD the applied voltage distorts only the bulk texture; it is close to two or three times the Freedericksz transition voltage, the minimum voltage to distort the texture maintained by the nematic elasticity. The breaking of the anchoring, taking into account the condition of the texture stability, needs a voltage almost ten times of the Freedericksz transition voltage.        4) The viscosity and the elastic constants of the mixture determine the optical response time of the displays. In the case of selection of the state by hydrodynamic coupling, these two parameters are fundamental also for the commutation.        5) A high optical refractive index anisotropy (0.14 to 0.20) has to be obtained to achieve the good contrast with a cell thickness smaller than in the classical LCD display. For a bistable display using anchoring breaking with a given mixture and a given anchoring, the voltage to break the anchoring is proportional to the thickness: to lower the voltage a small thickness is compulsory.        6) The nematic temperature range has to be wider than the targeted operating range. Indeed often the whole set of properties listed just before are not satisfied in the whole nematic range of the liquid crystal mixture: the range (ΔTN) limited by TN-I (the nematic-isotropic transition temperature) and TX-N (the transition temperature towards the nematic phase from more ordered liquid crystal phases or vitreous or crystalline solid phases). To obtain all these properties, in a technically acceptable temperature range (50° to 80° centred on the ambient temperature), the mixtures need to have a nematic temperature range ΔTN wider than this operating temperature range.        
The anchoring and anchoring breaking concepts for liquid crystal molecules on surfaces are highly technical, and they can be defined. The orientation of the liquid crystal molecules by surfaces is called anchoring. The source of anchoring is anisotropy of the interaction between the liquid crystal compound and the surface. Anchoring can be characterized by the directionality induced by the surface to which the liquid crystal molecules are adjacent and the strength thereof. This direction is called the easy axis, and the direction n0 of the easy axis is defined by the azimuth angle (Φ0) and the zenithal angle (θ0) (see FIG. 2). The average orientation direction of nematic liquid crystal molecules is drawn towards the easy axis. If there is no external influence, the liquid crystal molecules are oriented parallel to the easy axis to minimize the interaction energy with the surface. This energy (anchoring energy) may be written as the following equation formula (1) as a first approximation (A. Rapini and M. Papoular, J. Phys. (Fr) C4, 30, 54-56 (1969)):
                              g          ⁡                      (                          θ              ,              ϕ                        )                          =                                                            W                Z                            2                        ⁢                                          sin                2                            ⁡                              (                                  θ                  -                                      θ                    0                                                  )                                              +                                                    W                A                            2                        ⁢                                          sin                2                            ⁡                              (                                  ϕ                  -                                      ϕ                    0                                                  )                                                                        (        1        )            where θ and Φ are the zenithal angle and the azimuth angle, respectively, of the nematic director on the surface, and WZ and WA are the surface densities of the zenithal and azimuth anchoring energies, respectively.
The azimuthal anchoring energy WA depends more on the anisotropy induced on the surface by treatments, than on the nature of the nematic materials. Even if it is compulsory in a bistable liquid crystal display to achieve a sufficient azimuthal anchoring to maintain twisted textures, we will not develop this subject.
The bistable displays commuting by anchoring breaking use more often the zenithal anchoring breaking. We will focus on this phenomenon.
The zenithal anchoring energy WZ depends strongly on the chemical properties of the surface and of the nematic material. On most solid surfaces, the zenithal anchoring energy is one or two orders of magnitude higher than the azimuthal anchoring energy.
If the orientation of the director in the volume is different from the direction of the easy axis, the surface energy is no longer zero and the result dependents also on a bulk elasticity factor. The surface energy can be characterized by its extrapolation length that is the ratio between the bulk elasticity factor and the anchoring energy. The extrapolation length of the zenithal anchoring is denoted by LZ=K33/WZ, where K33 is the bend elastic constant of the liquid crystal. In practice, a zenithal anchoring is considered as being strong if LZ<15 nm and weak if LZ>25 nm.
The orientation of liquid crystal molecules may be modified by external electrical or magnetic fields. For example, by applying an electric field that is perpendicular to the surface of the substrate, when liquid crystal molecules in the cell have positive dielectric anisotropy, they are oriented along the field (θ=0), and in the absence of an electric field, they orient to be nearly parallel to the surface of the substrate (θ=approximately 90°. On the surface, the director zenithal angle changes continuously as a function of the field, and θ becomes zero if the field exceeds the critical field EC. This state is called zenithal anchoring break, since the director of the liquid crystal molecules close to the surface is no longer affected by an anchoring torque or an electric torque. The critical field is shown as the following equation formula (2):
                              E          C                =                              W            Z                                                              K                33                            ⁢              Δɛ                                                          (        2        )            
In the formula (2), WZ is the zenithal anchoring energy, K33 is the bending elastic constant, and Δ∈ the dielectric anisotropy (relative to the dielectric constant ∈0 of a vacuum).
This critical field strength Ec is the field is necessary to drive devices that utilize zenithal anchoring breaking. Mixtures with a high value of Δ∈ and high bending elasticity but a weak zenithal anchoring energy are necessary to control such devices.
In practice, the useful magnitude, in the case of zenithal anchoring breaking displays, is the voltage Uz that causes zenithal anchoring breaking. That is, the product of the critical field and the thickness of the cell. Usually, the thickness of bistable nematic displays cells is adjusted so that their birefringence is equal to the half wavelength of light at the center of their passband. To characterise the zenithal anchoring, the breaking voltage threshold Uλ/2 is used. It is the breaking voltage Uz of a cell of optical thickness λ/2. Uλ/2 is shown as the following equation formula (3).
                              U                      λ            /            2                          =                                            λ              ⁢                                                          ⁢                              W                Z                                                    2              ⁢                              Δ                ⁢                n                            ⁢                                                                    K                    33                                    ⁢                  Δɛ                                                              =                                    λ                              2                ⁢                                                      Δ                    ⁢                    nL                                    Z                                                      ⁢                                                            K                  33                                Δɛ                                                                        (        3        )            
In the formula (3), λ is the wavelength of light at the center of the passband, Wz is the zenithal anchoring energy, Lz is the zenithal anchoring extrapolation length, Δn is the refraction anisotropy in the wavelength λ, K33 is the bending elastic constant, and Δ∈ is the dielectric anisotropy. The inventors consider that zenithal anchoring is weak when the breaking voltage Uλ/2 is a voltage that can be supplied, within the temperature range, by a driver that is currently used ordinarily. In practice, this can be represented by the empirical rule stating that anchoring is weak if Uλ/2 is less than or equal to 25 volts.
The zenithal anchoring energy depends on the material of the alignment film, the method of the surface treatment, the liquid crystal composition used, and the temperature. The nature of the alignment film can greatly influence the zenithal anchoring energy. The polyimide orientation films used in classical LCDs show strong zenithal anchoring energy for most of the different families of nematic compounds. For example, on a commercially available polyimide orientation film (SE140 made by Nissan Chemicals Co.), LZ=7 nm for the nematic compound pentyl-cyanobiphenyl (5CB), zenithal anchoring is strong. On the other hand, Nemoptic Ltd. has developed copolymer films that provides weak zenithal anchoring for 5CB (LZ>25 nm at 20° C.) and for other nematic compounds (European Patent Application, Publication No. 1 259 854 and U.S. Pat. No. 7,067,180). In both patent documents, by standard method such as rubbing, a medium or strong azimuthal anchoring is simultaneously obtained leading to a good stability for both T0 and T180 textures.
Japanese Unexamined Patent Application, Publication No. 2005-133057 discloses an example of a liquid crystal composition in which the zenithal anchoring is weak. By combining nemoptic copolymer films with specific liquid crystal compositions, a low Uλ/2 is obtained, and combinations having a nematic phase over a wide temperature range are also disclosed. In fact optimized anchoring properties of bistable displays depend on both liquid crystal alignment layer and liquid crystal mixture, but some liquid crystal mixtures can lead to good anchoring properties compatible with different types of alignment layers.
However, in a bistable nematic display that uses zenithal anchoring breaking, the temperature range of the nematic phase and the operating temperature range of the display are not in a proportional relation. That is, at room temperature, even when the operating voltage is low, if the temperature dependency is high, the operating temperature range is in effect narrowed. In reality, a bistable nematic display that can operate over a wide temperature range has not been achieved. There is, therefore, a need to find a combination of liquid crystal compositions that can be activated over a wide operating temperature range.